Frequency selector system



J. L.`HURFF March 2l, 1939.

FREQUENCY SELECTOR SYS TEM Filed July 2l, 1936 m o. r 52.55 Bzwnow |052.m .253255. Somma o J SEPH L. HU

A TTORNEY.

Patented Mar. 21, 1939 UNITED 'STATES PATENT OFFICE Joseph L. Huri,Flushing, N. Y., assignor to Hazeltine Corporation, a corporation ofDelaware Application July 52,1,y 1936, Serial No. 91,667

3 Claims.

This invention relates to modulated-carrier signal receivers, and moreparticularly to signalselecting systems of such receivers and means forcontrolling the selectivity thereof to discriminate against undesiredsignals and to obtain optimum fidelity of reproduction consistent Withsignal reception conditions. While the vinvention is of general utility,it is particularly applicable to radio broadcast receivers of theWell-known superheterodyne type. y

The factors governing the optimum selectivity of the receiver are thesignal-to-noise ratio of the signal being received, which ratio depends,under normal conditions, on the strength of the desired signal carrierand on the presence of strong undesired signal carriers on channelsadjacent or near that of the desired signal. In general, a receivershould be effective to pass a band of desired modulation frequenciessufficiently narrow greatly to attenuate undesired signals, as, forexample, those transmitted on channels adjacent that of the desiredsignal. Such narrowing of the selected band of irequencies, however,tends to impair the delity of reproduction of the desired signal sincethe outer frequencies of the sidebands, which in radio broadcastingcorrespond to the higher audio irequencies of modulation, aresuppressed. Accordingly, it is desirable that the Width of the selectedband of frequencies be contracted only when disturbing noise or anundesired signal of sufficient amplitude to cause interference arepresent, and that, in their absence, the Width of the selected bandshould be expanded sufficiently to admit and pass all of the sidebandfrequencies of the desired signal.

Various .expedients have been proposed for obtaining the type ofselectivity control described above. In certain of these arrangementsadjustable impedance elements, such as vacuum tubes, have been connectedin circuit with the selective circuits adjustably to damp such circuitsand thereby adjust the width `of their band-pass characteristics. Thistype of element, however, is usually inherently of high impedance whichrestricts its applicationfto a parallel connection with the otherelements of the associated tuned circuit. As thus connected, theinherent capacitance of such an element incidentally influences theresonant frequency of the circuit in which it Vis connected. This effectis undesirable, particularly in circuits, such as radio-frequencypreselector circuits, which are tunable over a range oil frequencies,since it tends to restrict th tuning range. c

The main disadvantages of using vacuum tubes for variably damping tunedcircuits are realized most severely in association with the tunableinput circuit of a radio receiver. Both desired and undesired signalsmay be present in this circuit with substantial amplitude suiiicient tocause uctuation of the vacuum-tube shunt conductance with resultingdistortion of the desired signal or cross modulation thereof by theundesired signal. The latter is a form of interference Which cannot befiltered out in subsequent selective circuits. In the case of the usualinput circuit comprising fixed inductance and variable capacitance fortuning, the band Width thereof is affected unequally by the same shuntconductance for diierent tuning adjustments so that uniform increment ofband Width is not obtained.

In general, the use of nonlinear impedance elements, such as vacuumtubes, tends to result in undesired cross modulation effects. it isdesirable to provide in such selector circuits an impedance element ofvariable resistance having for any given value of resistance a linearrelation between voltage and current at high irequencies. Control isfacilitated if the resistance of such a device is subject to variationin response to adjustment of a control current or voltage applied to thedevice.

In the case of a superheterodyne receiver, one form of proposedselectivity control is obtained by connecting impedance elements incircuit With the resonant circuits of the intermediate-frequency channeland providing control means for adjusting the resistance oi theseelements to eX- pand the Width of the frequency band transmitted bythese circuits in accordance with the amplitude of theintermediate-frequency carrier. In providing a selectivity controlarrangement for this type of receiver, it is desirable not only toprovide means for adjusting the band-pass characteristic of theintermediate-frequency channel, but also to provide means for similarlyadjusting the Width of frequency bands passed by the radio-frequencyselector circuits of the receiver, by the same amount, independent ofthe tuning adjustment.

It is an object of the present invention to provide an improved simpleand economical arrangement which may easily be adjusted to control thesensitivity and selectivity of a modulated-carrier signal receiver sothat there may be obtained uniform output with maximum delity ofreproduction consistent With any particular condition of reception.

It is a further object of the invention to pro- Accordingly,

vide an arrangement for adjusting the width of the frequency bandtransmitted through a radiofrequency selector circuit with freedom fromundesired cross modulation of the desired signal carrier by strongundesired signals on nearby frequencies.

It is a further object to provide such an arrangement for aradio-frequency circuit tunable by variable capacitance in which theadjustment of the band width is the same for all tuning adjustments.

It is a further object of the invention to provide, in a superheterodyneradio receiver, a selectivity control arrangement of the characterdescribed above which may be adjusted to vary, by the same amount, thewidth of the frequency band to be transmittedthrough one or both of theradio-frequency and the intermediate-frequency channels of the receiver.

Briefly, the above objects are attained in accordanc'e with the presentinvention by providing, effectivelyin circuit with each of certain ofthe resonant frequency-selective circuits included in one or both oftheY radioand intermediatefrequency `channels of a radio receiver, aresistance element having a substantially constant resistance at highfrequencies, but a substantial variation of resistance with respect totemperature, that is, a substantial temperature coefficient ofresistance and controlling the temperature of these resistance elementsby means of a temperature control heating current of adjustablemagnitude to vary the selectivity of the several circuits. The inventioncontemplates, in particular, the use of such a device in the iirstradio-frequency selector circuit of the receiver for increasing the bandwidth and decreasing the selectivity of this circuit during reception ofstrong signals less subject to interference from strong undesiredsignals located on channels` near that of the desired received carrier,which undesired signals would tend to produce cross modulation effects;

In the preferred arrangement, the control current is varied directlyinaccordance with the amplitude of a desired signal carrier so that theresistance of each of the circuit damping resistance elements isincreased with increasing desired signal carrier amplitude to broadenthe frequency band of the associated circuits. A tungsten filamentincandescent lamp is one form of resistance element having the desiredcharacteristics.

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, willbest be understood by reference to the specification taken in connectionwith the accompanying drawing, in which Fig. 1 illustrates one form ofthe improved selectivity control arrangement in its application to aradio broadcast receiver of the superheterodyne type; and Fig. 2illustrates a modification of one of the band-pass selector systemsincluded in the receiver of Fig. 1.

Referring now more particularly to Fig. 1 of the drawing, thesuperheterodyne receiver there shown comprises a radio-frequency ampliertube I0 coupled by means of a tunable band-pass input circuit II to aninductance I2, the terminals of which comprise the input terminals ofthe receiver and are adapted to be connected to an antenna I3 and groundI 4. The output circuit l5 of the tube Ill is coupled to the tunableband-pass input circuit I6 of a frequency changer I1 comprising amodulator tube I8 and a local oscillator indicated schematically at I9in the cathode circuit of the tube I8. The oscillator i9 may be of anyform well known in the art and, since the details thereof form no partof the present invention they have been omitted from the drawing. Thevariable condensers of the resonant input circuits II and I 6 and thatincluded in oscillator I9 are preferably ganged for unicorntroladjustment, as indicated by the dotted line U. A The frequency changerI'I is coupled to an intermediate-frequency amplifier 2B by means of aband-pass selector system, indicated generally at 2I, which isadjustable to vary the width of the transmission band in a manner to bedescribed hereinafter. The output from the intermediate-frequencyamplifier 20 Yis coupled by means of a transformer 22 to a detector andY A. V. C. rectifier, indicated generally at 23, which comprises a dioderectifier tube 24 having itselectrodes connected across the secondary ofthe transformer 22rthrough a load circuit comprising series-connectedresistors 24 and 25 shunted respectively by carrier-frequency by-passcondensers 26 and 2l, -the junction points between the resistors 24 and25 and the condensers 26 and 2l being grounded asindicated at 29.Coupled in cascade With the detector 23 are an audiofrequency'amplifler30 and a sound reproducer SI. Suitable sources of operating potentials32, 33, 34, and 35 are provided for the tubes IG and I8.

Automatic amplification control is secured in a well-known manner byapplying the rectiiied voltage developed across the load resistor 25 asa negative bias on the control electrodes of one or more Vof the tubesincluded in the radio-frequency and the intermediate-frequency channelsof theV receiver-through a` connection '36 and high-irequencyv blockingresistors 3l. The audio-frequency voltage developed across the loadresistor 24 is applied to the input circuit of the audio-frequencyamplifier 30 by a connection 31,.

Neglecting for the present the operation of the selectivity controlmeans, the apparatus just described constitutes, in general, aconventional superheterodyne 'radio receiver, the operation of which iswell understood in the art. In brief, signals, intercepted by theantenna-ground circuit I3, I4, are selected and amplified in theselector circuit I I and the amplifier I 0, are further selected in theselector'ci'rcuit I6, and are converted to an intermediate-frequencysignal in the wellknown manner in the tunable frequency changer I1. Theintermediate-'frequency signals are further selected in` the selectorsystem 2|, are ampliiied in the intermediate-frequency amplifier 2B, andare delivered to the rectifier 23 wherein the audio-frequency signalsare derived. These signals are, in turn, Yamplified in the ampliiier 30and supplied to the loud-speaker 3| for repro-F frequency selectorcircuits I I and I6, means'for adjusting the width of the frequency bandpassed by the intermediate-frequency selector system 2 I, and controlmeans 38 for simultaneously controlling the two band width adjustingmeans. The band width adjusting means for the tunable selector circuit II comprises a resistor 39 connected in series with the other elements ofthe u circuit by means of direct current blocking condensers 48 and 4I.Similarly, a resistor 42 is connected in series with the other elementsof the tunable selector circuit I6 through direct current blockingcondensers 43 and 44.

The width of the frequency band transmitted throughr theintermediate-frequency channel of the receiver is determined by theadjustment of the selector system 2|. This system comprises a pluralityof resonant circuits including an input circuit I, an output circuitIII, and an intermediate link or coupling circuit II. Each of thesecircuits comprises one or more inductance elements 45 and one or moretuning condensers 45, by means of which the circuits are individuallytuned to the intermediate frequency of the receiver. Inductance elements45 of the circuits I and II are inductively coupled, as are thecorresponding elements 45 of the circuits II and III. The couplingsbetween the circuits I and II and between II and III are initiallyadjusted in accordance with well-known design principles to obtain Athemaximum desired transmission frequency band with maximum resistance of41 and minimum resistance of 48. The resistor 41 is connected in serieswith the elements 45 and 46 of the circuit I and the resistor 48 isconnected in series with the elements 45 and 46 of the link circuit II.

Preferably, each of the enumerated resistors 39, 42, 41, 48 comprises anincandescent lamp including a lament of tungsten or like material havinga large temperature coeiiicient of resistance such that, when it issubjected to a variable current, it manifests a substantial currentcoef- Lcient of resistance. The filaments of these resistors aredesigned with suicient thermal incr- A tia that their resistances aresubstantially unaffected by radio-frequency or intermediate-frequencyvariations of current therein. As was noted previously, this form ofresistor has certain desirable characteristics when used in a selectorcircuit as a damping means.

In order to adjust the resistance values of the resistors 39, 42, and 41simultaneously in the same sense, these resistors are connected inseries by means of high-frequency choke coils 49 and 58. These coils andthe by-pass condenser 4I, 44 serve to isolate the selector circuits fromeach other for currents of high frequency. Similarly, the blockingcondensers 49, 4|, 43, 44 are eiective to prevent direct current fromiiowing through the elements of the selector circuits Ii and I6 otherthan the resistors 39 and 42. Control current is supplied to theseseries-connected resistors by the voltage sources 34 and 35 and theamplitude of this current is controlled by a tube I comprising a part ofthe control means 38 and having its space-current path included inseries with these resistors. The voltage sources 34 and 35 also supplycurrent to the resistor 48, the magnitude of which is controlled by atube 52 also included in the control means 38 and having itsspace-current path in series with this resistor. The latter spacecurrent is varied to adjust the resistance of 48 in the opposite senserelative to the other three resistors, as required by the selector 2|.

Preferably, thev control means 38 is arranged automatically to controlthe magnitude of current through the several resistors in accordancewith the amplitude of a desired received carrier in such manner that thewidth of the frequency band transmitted through the selector circuitsII'and I6 and the system 2| is varied directly with the intensity of thesignal carrier. To this end the control electrode of the tube 5| isconnected through an audio-frequency blocking resistor 53 and theconnection 31 to the positive terminal of the load resistor 24, and thecontrol electrode of the tube 52 is connected through an audio-frequencyblocking resistor 54 to the automatic volume control connection 36. Thecontrol electrodes of the tubes 5| and 52 are bypassed to ground foraudio-frequency currents by condensers 55 and 56, respectively.Auxiliary manually adjustable means are provided for further controllingthe effective bias applied between the input electrodes of these tubes.For the tube 5|, this means comprises a voltage source 51 shunted by avoltage divider 58 having an adjustable arm` 59 connected to the cathodeof the tube and, for the tube 52, it comprises a voltage source 60shunted by a voltage dividing resistor 6I having an adjustable arm 62connected to the cathode of the tube.

In considering the operation of the selectivity control means justdescribed, it will be assumed that, in the absence oi a received carrierand with the auxiliary control means 38 properly adjusted, the biasapplied positively to the control tube 5| through the connection 31 is aminimum and the space current flowing through this tube and through theresistors 39, 42, and 41 is adjusted to its minimum value. Thus, theseresistors are adjusted to their minimum resistance values, such that thedamping of the respective circuits I I, I6 and I is minimum. As aconsequence, these circuits are adjusted to pass the minimum band offrequencies. Under these conditions, the bias applied negatively to thecontrol tube 52 is also a minimum and the space current flowing throughthis tube and through the resistor 48 is adjusted to its maximum valuecorresponding to the maximum resistance value of this resistor. Hence,the damping of circuit II is a maximum and the effective couplingbetween the circuits I and III is minimized for minimum transmissionband of the selector 2|.

With gradually increasing signal strength, the effective positive andnegative bias voltages applied respectively to the control electrodes ofthe tubes 5I and 52 are increased simultaneously to increase the currentflowing through the resistors 38, 42, and 41 and to decrease the currentflowing through the resistor 48. As a result, the resistances of theresistors 39 and 42 are gradually increased to increase the width of thetransmission band through the circuits II and I6; the resistance of theresistor 48 is gradually decreased to increase the effective couplingbetween the circuits I and III and thereby gradually to increase thewidth of the transmission band of the selector 2 I; and the resistanceof the resistor 41 is gradually increased to increase the damping of thecircuit I. The resistor 41 serves to provide proper damping in theentire selector 2| and thereby to assure a flat-top resonance curve oruniform transmission characteristic therefor. It is initially adjustedwith such a maximum value as to meet these requirements when the systemis adjusted to provide the maximum desired band width. By simultaneouslyadjusting the resistor 48, in the manner described, to increase thelosses in the circuit II and the resistor 41 to decrease the losses incircuit I, a proper relation between damping and coupling in theselector 2| is maintained for all band width adjustments and the desiredtransition from broad band to sharp selectivity characteristics may beobtained.

ganged for unicontrol adjustment, such that one bias voltage isincreased, While the other is being decreased. The maximum value of eachbias is preferably of the order of the value required to cut off thespace current in the'corresponding tube.

Referring now more particularly to Fig. 2 of the drawing, there isillustrated a modification of the selector system 2l included in theintermediatefrequency channel of the receiver, which is ad- `iustable tovary the Width of the transmission band through this channel. Thearrangement comprisesl an input circuit IV, an output circuit V coupledto the intermediate-frequency amplifier 2U, and a tertiary or absorptioncircuit VI for controllingr the band Width of the selector as a whole.Each of the enumerated circuits comprises a parallel connected inductor63 and condenser 64, and is permanently tuned to the intermediatefrequency; The remaining portions of the receiver may be similar in allrespects to that represented by Fig. 1. In order to vary the effectivedamping of the absorption circuit VI, a resistor 65 of the typedescribed above is connected in series with the eelments 63 and B4 ofAthe circuit VI and a control current of adjustable magnitude isconducted through this resistor through the terminals 61 and 68.

The circuits IV and V are coupled by less than optimum-coupling and thecircuits V and VI have more than optimum coupling. The resistor 66 ischosen of a resistance Value such that the maximum desired band Width isobtained when the resistor has its minimum resistance value. The maximumresistance of 66 should be suiioient toy make negligible the absorptioneffect of VI and thereby to secure minimum band width.

In the operation of the system of Fig. 2, to control the band Width oftransmission through the intermediate-frequency channel of the receiver,an increase in the resistance of the resistor 66, in response to an`increase in the magnitude of the control current applied thereto through`the terminals '6l' and 68, increases the damping of the circuit VI todecrease the Width of the frequency band passed by the system 2|.Conversely, a decrease in the resistance of the resistor 66, to decreasethe damping of the circuit, increases the Width of the frequency band.The latter operation has the effect o-f lowering only the center part ofthe resonance curve to increase the effective band Width. It is desiredto have heating current in 66 which decreases with increasing carrieramplitude of the desired signal, as in. the resistor 43 of Fig. 1. Thismay be obtained by the substitution of the selector 2l of Fig. 2 forthat of Fig. 1, so that circuits IV, V, VI replace circuits I, III, II,respectively, the terminal 61 is connected to the positive terminal ofthe voltage source 35, and the A. V. C. lead'is connected to theconnection Sli.

Particular reference is made to the series connection of the temperaturecontrolled resistors 39 and 42 inthe tunable radio-frequency circuits Hand I6 in Fig. .'1. These .circuits have fixed inductan'c'e and`variable capacitance for tuning. It is a` property of such` a circuitthat a given increment of series resistance increases the band width ofthe circuit by an amount Which does not vary with tuning. The band widthof the intermediate-frequency selector 2l also does not vary with thetuning of the receiver. Therefore, it is possible and preferable tosecure approximately equal band Widths of the tunable selectors ll, IGandthe tuned selector 2l, and to maintain this relation with variationof the controlled resistors during expansionl of the band Width.

The essential property of the temperature controlled resistors 39, 42,4l, 48, 66 is thermal inertia sufficient to prevent appreciableresistance fluctuation at high frequencies and, preferably, also ataudio frequencies. The variation of temperature may be secured byheating current in the controlled resistor, or in a heating elementclose to the resistor. ing which is advantageous Where large thermalinertia is desired, or Where the heating circuit must be isolated fromthat of the resistor. The present invention contemplates the use ofeither direct or indirect heating of the resistor by the controlcurrent. Y While there has vbeen described what is at present consideredto be the preferred embodiment of the invention, it Will be obvious tothose skilled in theart that various changes and modifications may bemade therein without departing from the invention, and it is, therefore,aimed in the appended claims to cover allsuch changes and modificationsas fall Within the true spirit and scope of the invention.

What is claimed is:

l. In a superheterodyne radio receiver including at least one resonantcircuit tunable over a range of frequencies to select a desired signalcarrier, a tunable frequency changer and at least one resonant circuitpermanently tuned to the intermediate frequency, an electric circuitarrangement for controlling the frequency transmission band Width ofsaid circuits comprising an element connected in each of said resonantcircuits having resistance variable with temperature to vary the Widthof the frequency band passed by the rassociated circuit and means foradjusting the temperature of said elements, said resistance elements ofsaid tunable and tuned circuits being proportioned and the resistancesthereof being adjusted by said adjusting means to cause approximatelyequal variation of the effective band Width of the individual tunableand tuned circuits.

The latter effects indirect heatv Cil 2. In a superheterodyne radioreceiver including at least one resonant circuit tunable over a range offrequencies to select a desired signal carrier, a tunable frequencychanger and at least one resonant rcircuit permanently tuned `to theintermediate frequency, an electric circuit arrangement for controllingthe frequency transmission Wiclth of said circuits comprising an ele- 3.,In a superheterodyne radio receiver including at least one resonantcircuit comprising xed inductance and capacitance variable to tune saidcircuit over a range of frequencies to select a desired signal carrier,a tunable frequency changer and at least one resonant circuitpermanently tuned to the intermediate frequency, an electric circuitarrangement for controlling the frequency transmission band width ofsaid circuits comprising an element connected in series with the otherelements of each of said resonant circuits having resistance variablewith temperature to Vary the width of the frequency band passed by theassociated circuit and means for adjusting the temperature of saidelements, said resistance elements of said tunable and tuned circuitsbeing proportioned and the resistances thereof being adjusted by saidadjusting means to cause approximately equal Variation of the effectiveband width of the individual tunable and tuned circuits.

JOSEPH L. HURFF.

